Powdery Mildew: What Causes It and How to Get Rid of It

Powdery mildew is a fungal disease that shows as white, flour-like patches on the leaves. It is common in grow rooms and greenhouses, and far easier to keep out than to shift. Here is how to spot it, tell it from grey mould, keep it out and get rid of it.

What powdery mildew looks like

• White or pale grey spots, usually on the upper side of a leaf. They rub off, then come back.
• The spots join into patches and spread to other leaves, stems and flowers.
• Badly affected leaves yellow, dry up and drop.

It often starts low down and inside the plant, where the leaves are shaded and the air moves least. Dried spray and hard-water marks look similar but do not spread.

Powdery mildew or grey mould (botrytis)?

Grey mould (botrytis) is a different fungus: a fuzzy grey-brown rot that usually starts in dense flowers, soft fruit and damaged tissue, where powdery mildew is a dry white coating on the leaf. Both like still, humid air.

Cut out the rot and a little clean growth beyond it, bag it beside the plant and do not compost it, then wash your hands and scissors. After that, keep flowering humidity at 40-55% (see Prevention below and our humidity guide), above all in the dark period late on. If extraction cannot keep up, add a dehumidifier; the G.A.S Day & Night Controllers come in a dehumidifier version, and their page says they adjust settings to the light cycle. The Hotbox Sulfume page says it is designed to stop the spread of fungal infections; late in flowering, though, rely on cutting out and drier air (see Sprays and additives compared below).

What brings it on

The spores travel on the air and on clothes and tools, so most rooms meet them sooner or later. Unlike most moulds, powdery mildew does not need wet leaves. What helps it along:

• Still, humid air. Damp air that sits around the leaves helps the spores take hold. See our airflow guide.
• Crowded plants. Overlapping leaves trap damp air where no fan reaches.
• Big day-night swings. When the lights go off and the room cools, relative humidity climbs.
• New arrivals. It commonly comes in on cuttings and plants from elsewhere, so check them first.

Prevention: keep the humidity down

• Measure it with a hygrometer. The ThermoPro TP358 records the highest and lowest humidity over 24 hours and has high and low alarms.
• Set a target. Our airflow guide gives 55-70% relative humidity in vegetative growth and 40-55% in flowering. At 24°C, about 50-55% also sits inside the flowering range on our VPD chart.
• Extract enough air, and keep the extractor running through the dark period, when humidity peaks. See how to size a fan and carbon filter.
• Add a dehumidifier when extraction cannot keep up: a sealed room, damp weather, big plants late in flowering.
• Soften the night-time drop with a little background heat: the Hylite Tube Heaters page says they help reduce mould and mildew by keeping the room drier.

Our humidity guide compares the dehumidifiers we sell, from the smaller RAM units to the overhead Quest units, and gives a rule of thumb for sizing one.

Prevention: keep the air moving

• Clip fans move air between and under the leaves. The Maxifan Clip Fan 200mm has an adjustable head; AC Infinity Cloudray fans come in 100, 150 and 225 mm.
• Add an oscillating wall fan. The WindKing Wall Fan swings side to side and up and down; its page says uniform conditions mean less chance of rot and powdery mildew.
• Give the plants room. Bendz clips bend stems apart and the ProNet 120 support net keeps plants separated.

What to do when you see it

1. Check every plant, low down and inside.
2. Take off the affected leaves. Bag them beside the plant so spores are not shaken over the rest, then wash your hands and scissors. A plant that is covered is better taken out of the room.
3. Fix the air. Lower the humidity, point a fan through the plants and thin crowded growth, or the mildew is likely to return whatever you spray.
4. Spray, if you choose to, covering both sides of every leaf. The Athena IPW page recommends a pump sprayer (the Garden HighPro Pumpro Sprayer 2L is one), with the fans off for even coverage. Wear the protective gear the label asks for, and turn the fans back on afterwards so the leaves dry.
5. Keep checking every few days.

Sprays and additives compared

Going by those pages, CannaCure and Solar Green Power are for prevention; the other three are also for use once mildew shows.

Whatever you use, read the label first: the dose, how often to repeat it, whether it can go on plants you'll eat, and how close to harvest you can use it (the Athena IPW page says up to week 5 of flowering). Late on, rely on removing leaves and drier air.

The Hotbox Sulfume uses sulphur, not a spray. As a rule, sulphur and oil-based sprays should not be used close together, as the combination can scorch leaves. Follow the full instructions supplied with the Sulfume. See all Pest Control.

Cleaning between crops

Mildew can carry over from one crop to the next, most easily on living plants, so check any mother plants or cuttings you are keeping, and clean while the room is empty.

• Clear out old leaves, roots and growing medium.
• Wash pots, trays and tools, and wipe down walls, floor and fan blades (see cleaning products). Silver Bullet Mist is a stabilised silver and hydrogen peroxide disinfectant; its page says it eliminates bacteria, viruses and fungi on contact.

Quick answers

What causes powdery mildew?

A fungus. Its spores arrive on the air or on new plants and take hold in still, humid air among crowded leaves.

How do I get rid of powdery mildew?

Take off the affected leaves and bag them, lower the humidity and get air moving through the plants. A spray used as its label directs can help, but mildew tends to come back while the room stays damp and still.

More Growers' Guides

• VPD chart
• Airflow, temperature and humidity
• Fan and carbon filter size
• Fungus gnats

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.